In general, optical disk devices converge a light beam spot on an information layer in an optical disk, and receive reflected light from a track or pit formed on the optical disk to read information from the optical disk. At this time, tracking control is performed to allow the light beam spot to follow the track or pit, and focus control is performed to form the light beam spot on the track or pit in an appropriate converged condition.
Here, FIG. 10 is a block diagram showing the configuration of a detector and a signal processing section in an optical disk device according to a conventional technique; the detector detects a tracking error signal (hereinafter referred to as a TE signal) for tracking control and a focus error signal (hereinafter referred to as an FE signal) for focus control.
As shown in FIG. 10, a detector 108 has four equal detection areas A, B, C, and D into which the detector 108 is divided by cross partition lines. The partition line extending in a lateral direction of FIG. 10 corresponds to a radial direction (hereinafter referred to as a tracking direction) of the optical disk. The partition line extending in a vertical direction of FIG. 10 corresponds to a pit longitudinal direction of the optical disk. Preamplifiers 109a to 109d are electric elements that convert output currents from the areas A to D of the detector 108 into voltages.
An FE signal generating section 110 is an electric circuit that generates the FE signal corresponding to a converged condition of the light beam on the information layer in the optical disk, from output signals from the preamplifiers 109a to 109d by means of what is called an astigmatic focus error detection. A TE signal generating section 120 is an electric circuit that generates the TE signal corresponding to a tracking condition of the light beam on the information layer in the optical disk, from the output signals from the preamplifiers 109a to 109d by means of what is called a push-pull tracking error detection.
The FE signal generating section 110 is composed of an adder 110a that adds the signals from the preamplifiers 109a and 109c together, an adder 110b that adds the signals from the preamplifiers 109b and 109d together, and a subtractor 110c that subtracts an output from the adder 110b from an output from the adder 110a. 
Furthermore, the TE signal generating section 120 is composed of an adder 120a that adds the signals from the preamplifiers 109a and 109b together, an adder 120b that adds the signals from the preamplifiers 109c and 109d together, and a subtractor 120c that subtracts an output from the adder 120b from an output from the adder 120a. 
With the configuration described above, the optical disk device performs the focus error control and the tracking error control by generating the TE signal and the FE signal from the detected light from the single detector 108.
In the above-described optical disk device, when the light beam spot converged on the information layer in the optical disk traverses the track or pit, optical crosstalk may occur in which the TE signal leaks into the FE signal corresponding to the converged condition of the light beam spot.
As described above, the TE signal generating section 120 detects the TE signal using the push-pull tracking error detection. However, an adjustment residual or the like in the detector 108 for focus detection may cause mixture of a push-pull component (the amount of unbalance between both light receiving sections A and B and both the light receiving sections C and D of the detector 108).
When the optical crosstalk occurs, the optical beam spot is deflected in a direction perpendicular to the information layer in the optical disk (this direction is hereinafter referred to as a focus direction) owing to the focus control. If the deflection is significant, the focus control may fail.
To prevent such a failure in focus control caused by optical crosstalk, the optical disk device shown in FIG. 10 carries out signal processing on the basis of a configuration described below. That is, the amount of TE signal leaking into the FE signal while the tracking control is off is predetermined in the form of a setting signal. A multiplier 130 operating on the basis of the setting signal performs multiplication by a gain corresponding to the leakage amount to adjust the level of the TE signal output by the TE signal generating section 120. The signal with the level adjusted is input to a subtractor 150, which subtracts the level-adjusted signal from the FE signal to generate a corrected FE signal with the optical crosstalk corrected. The safety of the focus control is ensured on the basis of the corrected FE signal.
A conventional essential problem with the push-pull tracking error detection, used to detect the TE signal, is that a lens shift may cause offset. For example, in a lens shift condition in which an objective lens is shifted in a direction orthogonal to the optical axis of the light beam, return light of the light beam reflected by the information layer in the optical disk is received at a position shifted from the center of a light receiving surface of the detector 108. As a result, offset occurs in the TE signal.
When the lens shift has thus caused the offset in the TF signal, the corrected FE signal, utilizing the TE signal, also suffers the offset. This means that even though the light beam is in focus with respect to the information layer, the corrected FE signal has a value other than 0.
The focus control using the corrected FE signal with the offset makes the light beam out of focus with respect to the information layer. The recording and reproducing performance of the optical disk device is thus degraded.
To prevent the out-of-focus condition based on the erroneous focus control based on the corrected FE signal resulting from the offset of the TE signal, the above-described configuration has a high pass filter (hereinafter referred to as an HPF) 140 succeeding the TE signal generating section 120 and the multiplier 130 (see, for example, Japanese Patent No. 3567639 (for example, pp. 4 to 6)).
A push-pull signal, an output from the multiplier 130, passes through the HPF 140, which then removes a DC component corresponding to the offset. By generating a corrected FE signal on the basis of the TE signal with the DC component removed, the optical crosstalk can be, corrected with the possible out-of-focus condition prevented.
On the other hand, optical disks have been improved so as to increase the density and capacity thereof. The development of the optical disks started with CDs (Compact Discs) mainly intended to record music, text information, and the like and proceeded to DVDs (hereinafter referred to as DVDs) intended to record large-capacity information such as motion pictures. In recent years, Blu-ray discs (hereinafter referred to as BDs) with a further increased recording density have been proposed.
In particular, high-density optical disks typified by the BDs, next-generation disks, allow information to be recorded thereon at a higher density than optical disks conforming to conventional standards. Thus, the high-density optical disks involve a larger amount of optical crosstalk components contained in the reflected light from the information layer than the disks conforming to conventional standards, such as the DVDs.
Consequently, to allow the above-described conventional optical disk device to obtain the corrected FE signal corresponding to the optical crosstalk, it is important that the multiplier 130 accurately sets the gain based on the amount of leakage based on the TE signal. Moreover, the offset component contained in the TE signal needs to be reliably removed in order to accurately set the gain in the multiplier 130.
However, the present inventors have found that the following problems may occur if the high-density disk such as the BD is used in the above-described conventional optical disk device.
That is, if the HPF 140 is used to remove the DC component containing the offset component resulting from the lens shift, the frequency characteristic of the HPF 140 attenuates the offset component of the TE signal. However, the phase characteristic of a low frequency region inherently contained in the TE signal varies near the cutoff frequency of the HPF 140.
In this case, in the corrected FE signal, the phase characteristic of the low frequency region varies. Thus, disadvantageously, the optical crosstalk fails to be optimally corrected, resulting in an increase in power consumption and unstable focus control.
This problem is significant when the TE signal varies at low frequencies as in the case of tracking pull-in. However, in the high-density disk such as the BD, which involves frequent tracking in the same information layer and frequent interlayer movement as occurs in multiple information layers, the TE signal varies at low frequencies far more frequently than in the conventional optical disks. Thus, the presence and removal of the offset contained in the TE signal is an unignorable problem for the focus control involving the optical crosstalk correction.
The present invention has been made to solve these problems, and provides an optical disk device and the like which can accurately perform the optical crosstalk correction while dealing with the possible offset component in the tracking error signal.